1. Field
The present invention concerns a method of conditioning a wireless communication module's receive path to prevent transmit or other signals bled onto the receive path from modulating a relatively strong jammer (interference) signal via an amplifier or other nonlinear device, and possibly spreading the jammer signal into the frequency band occupied by relatively weaker receive signals of interest from remote stations. By combining the amplitude modulated bleed-over signal with a prescribed dummy signal, the nonlinear device in the receive path is forced to operate in a linear regime with respect to the jammer signal, leaving the jammer signal unmodulated (or, at least, free from added modulation). The dummy, bleed-over, and jammer signals (and any other signals generated upon introduction of the dummy signal) may then be filtered from the nonlinear device's output.
2. Background
A circuit is “linear” when it applies the same function to input signals regardless of the input signals' characteristics. For instance, a circuit is free from amplitude dependent nonlinearity if it applies the same function to input signals whether they have a small amplitude or a large amplitude. Conversely, a circuit exhibits amplitude dependent nonlinearity if its function changes according to the amplitude of the input signal. One example of a circuit with amplitude dependent nonlinearity is an amplifier that multiplies small amplitude input signals by ten, but with input signals of increasing amplitude, multiplies them by successively lesser numbers such 9.8, 9.7, 9.6, 9.5, and so on. The amplifier's behavior is therefore dependent upon the magnitude of its input signal.
Nonlinearity is an inherent property of many circuits as well as various circuits elements such as transistors, and it may even be desirable in different situations. In processing amplitude modulated communication signals, however, nonlinear circuit elements are definitely undesirable. Amplitude modulated signals, by definition, express information by the manner in which the amplitude of a signal's envelope varies. With this amplitude variation, nonlinear circuits therefore process amplitude variant signals inconsistently—the same function is not applied universally. One effect of this is that the input signal's frequency bandwidth is broadened. For example, an input signal that initially occupies a narrow frequency bandwidth ends up occupying a wider range of frequencies. Therefore, circuits with amplitude dependent nonlinearity often increase the bandwidth of amplitude modulated input signals.
This frequency spreading can cause problems. For example, a communication device's output signal, broadened by the nonlinear effect described above, may overlap into the frequency being used by another device of the same type. As a more particular example, a first cordless telephone's transmissions may overlap into the frequency channel being used by a second cordless telephone to receive. This is called “interference” and can significantly degrade the second phone's operation. Moreover, if a device is using a channel on the edge of the allocated frequency band for such devices, the device's transmissions may even overlap into the frequency band for unrelated devices. Thus, a cordless phone may interfere with a different device that is not even a cordless phone.
Furthermore, in an especially pernicious type of interference, a device may even interfere with itself. In order to permit users to simultaneously talk and listen, most communication devices transmit on one frequency but receive on a different frequency. In some transceivers, the signal being transmitted (“transmit signal”) to a remote station may inadvertently bleed over onto the receive path. The receive path often includes interference (“jammer signals”), which may be substantially stronger in amplitude than signals of interest (“receive signals”), which the transceiver is trying to receive from the remote station and process.
Necessarily, the receive path includes an amplifier because the receive signals are so weak. When the amplitude variant combination of transmit and jammer signals is fed to the receive amplifier, which is nonlinear across the range of varying amplitudes, the amplifier causes the jammer signal and transmit signal to experience frequency spreading. More particularly, the leaked transmit signal (having an envelope with varying amplitude) changes the gain of the amplifier. This has an effect on the jammer signal—if it was unmodulated, it is now modulated in a similar way as the transmit signal. If the jammer was originally modulated, it is now more modulated. This is called “cross-modulation,” since the amplitude modulation of the transmit signal transfers (crosses) to the jammer. Due to the proximity of the receive frequency to typical interference frequencies, the frequency-spread jammer signal can overlap onto the receive frequency. Thus, the strong jammer signal substantially overshadows the receive signals, making them difficult to discern.
A number of approaches can be used in an attempt to combat this effect. One idea is to filter the receive amplifier's output to remove unwanted signals. However, if the frequency-spread jammer signal now occupies the same frequency bandwidth as the receive signals of interest, filtering is useless because it would also filter-out the receive signal itself Another technique is to filter the transmit and jammer signals from the receive path before amplification. This technique is not entirely adequate because (a) the jammer is often too close to the receive signal to filter, and, (b), the transmit signal is also quite close, and expensive to filter since adequate good duplexers are large and expensive.
Consequently, due to certain unsolved problems, the receive signal processing of wireless transceivers is not always adequate for all applications.